1. Field of the Invention
The present invention relates generally to a fuel injection control system for an internal combustion engine. More specifically, the invention relates to a fuel injection control system which can precisely control air/fuel ratio of an air/fuel mixture actually combusted in a combustion chamber of the engine.
2. Description of the Background Art
In general, fluctuation of air/fuel ratio from a target or stoichiometric value is caused by variation of fuel amount consumed for wetting intake manifold, intake port and so forth and variation of amount of fuel suspended in the induction system. The fuel consumed for wetting the intake manifold and intake port will be hereafter referred to as "wetting fuel" and amount of fuel consumed as wetting fuel will be hereafter referred to "wetting fuel amount". On the other hand, the fuel suspended in the induction system will be hereafter referred to as "suspending fuel" and the amount of the suspended fuel will be hereafter referred to as "suspending fuel amount". The wetting fuel amount and suspending fuel amount are variable depending upon the engine driving condition. Variation of the wetting and suspending fuel amount are not linear or stepwise but non-linear in nature. Furthermore, variations of wetting fuel amount and suspending fuel amount occurs with a time delay which is not defined by a given time constant. In addition, the wetting fuel amount and suspending fuel amount vary not only according to the instantaneous engine driving condition but also according to difference between the instantaneous wetting and suspending fuel amount and the wetting and suspending fuel amount in the steady state. Therefore, dynamic characteristics of a fuel system using the induction system is indeterminate since part of injected fuel may be consumed as wetting fuel and part of fuel on the periphery of the induction system is vaporized to be introduced into the combustion cylinder with the injected fuel. Therefore, it is difficult or rather impossible to precisely control the air/fuel ratio at the stoichiometric value.
Japanese Patent First (unexamined) Publication (Tokkai) Showa 60-166731 discloses a fuel injection control system. In the disclosed system, the wetting fuel amount is assumed or projected on the basis of the transition period of an oxygen concentration indicative signal produced by an oxygen sensor provided in an exhaust system, which transition period varies according to engine speed. The fuel injection amount is controlled on the basis of the assumed or projected wetting fuel amount so as to maintain the air/fuel ratio near the stoichiometric value for anti-pollution purposes.
When feedback control of the air/fuel ratio is performed for controlling fuel injection amount on the basis of the oxygen concentration indicative signal of the oxygen sensor in the engine which has relatively large amount of wetting fuel amount and fuel vaporization amount, hunting in fuel injection amount control can occur, making air/fuel ratio worse.